ABSTRACT: NH4+ and NO3- seasonal uptake by the phytoplankton and <2-µm communities were investigated in the Menai Strait using 15N tracer techniques. Based on O2-determined primary production and DIN metabolism measurements, five phases were defined: (1) prebloom (before April), (2) primary net autotrophic (April and May), (3) net heterotrophic (June), (4) secondary net autotrophic (July and August), and (5) regenerative (remainder of calendar year). The inorganic N nutrition of phytoplankton and <2-µm organisms was NO3--based in spring and NH4+ -based in summer and autumn, reflecting varying ambient concentrations of these nutrients superimposed on a general preference for NH4+ by both communities. The percent contribution of picoplankton to the total NH4+ uptake was higher than to nitrate (seasonal averages of 50 ± 14% and 32 ± 24%, respectively). The contribution of the <2-µm size fraction was lowest during both net autotrophic phases (4-13%) and highest during the net heterotrophic, prebloom and regenerative phase (70-84%). A strong correlation between percent NH4+ and NO3- assimilation by the <2-µm size fraction and bacteria abundance between May and July suggests significant bacterial uptake at this time. Bacterial DIN utilization was estimated by correcting uptake rates by the <2-µm fraction, for picophytoplankton presence. During the primary net autotrophic phase, inferred bacterial uptake constituted 13 and 34% of total NO3- and NH4+ uptake, respectively. In the net heterotrophic phase, it amounted to ca. 70% of the total DIN uptake, with no differences between nutrients. Competition between bacteria and phytoplankton for depleted DIN could have contributed to the phytoplankton bloom termination.

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